[Boost-users] any_range vs. “canonical form” - what is the latter?

Boost's any_range documentation says the following: "Despite the underlying any_iterator being the fastest available implementation, the performance overhead of any_range is still appreciable due to the cost of virtual function calls required to implement increment, decrement, advance, equal etc. Frequently a better design choice is to convert to a canonical form." What does the author mean by a "canonical form"? Can someone give an example? Many thanks [question originally asked at http://stackoverflow.com/questions/5601303]